In recent years, along with wide spread of mobile appliances such as cell phones and high speed and high frequency of semiconductor devices which are main parts of personal computers, a multilayer ceramic capacitor to be disposed in such electronic appliances has been required to be smaller and have higher capacities. Therefore, a dielectric layer composing the multilayer ceramic capacitor has been made thinner and to have higher lamination.
For example, with respect to a dielectric powder composing a dielectric ceramic, Patent Document No. 1 discloses that a barium titanate powder (BCT (Barium Calcium titanate) powder) in which a portion of A site is replaced with Ca and a barium titanate powder (BT powder) containing no substituent Ca are used in form of a mixture, so that the fine granulation and relative permittivity improvement of the above-mentioned barium titanate crystal particles can be accomplished in the dielectric layer after sintering and at the same time DC bias property can be improved.
In this connection, when being mixed and sintered with added components such as Mg, rare earth elements, etc., which are indispensable for controlling temperature property of the relative permittivity, the BCT crystal particles among the barium titanate crystal particles composing the dielectric ceramic disclosed in Patent Document No. 1 tend to cause grain growth along with diffusion of Ca contained in BCT powder. Therefore, severe condition control is needed for sintering and particularly in the case of using raw materials having particle diameters of sub-micron level or smaller, it has been known well that rather considerable grain growth is caused and production of a sintered body composed of fine particles of barium titanate crystal is not easy.
Therefore, in the above-mentioned Patent Document No. 1, to suppress the grain growth of BCT crystal particles at the time of sintering, MnCO3, MgO, and a rare earth oxide are added further when a BT powder coated with oxides of Mg and rare earth elements and a BCT powder are mixed and accordingly, a coating layer of a highly insulating compounded oxide can be formed approximately evenly on the surfaces of the BT type crystal particles after sintering and at the same time excess solid solution formation and grain growth of Mg and the rare earth element in the BCT crystal particles.
According to the production method described in the above-mentioned Patent Document No. 1, in the case of using a small scale experimental sintering furnace in which the sintering temperature can be precisely controlled, even if a technique of further adding MnCO3, MgO, and a rare earth oxide is employed at the time of mixing the above-mentioned BT powder coated with Mg and a rare earth oxide and a BCT powder, it is made possible to form a sample which is provided with a desired relative permittivity and temperature property and satisfactory in a highly accelerated life test.
However, in the control level of the sintering temperature in a tunnel type large-scale sintering furnace just like the one to be used for mass production of multilayer ceramic capacitors, fluctuation of the highest temperature in the sintering furnace is considerably significant at the time of sintering and therefore, the dispersion of the grain growth of BCT crystal particles becomes wide and many products whose relative permittivity, temperature property, and highly accelerated life test property are out of the satisfactory ranges are produced to result in a problem that the yield of mass production is decreased.
Patent Document No. 1: Japanese Unexamined Patent Publication No. 2003-40671